Becoz "IPv6 packets are encapsulated directly behind the IPv4 header" so we can deduce an IPv4 header must be placed before an IPv6 header.
The IPv6 header is always present and is a fixed size of 40 bytes.
True about EIGRP
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-When summarization is configured, the router will also create a route to null 0
-Summarization is configured on a per-interface level
-Automatic summarization across major network boundaries is enabled by default.
-The value of the EIGRP variance command of 1 means the router performs equal-cost load balancing.
-Update packets route reliable change information only to the affected routers.
OSPF
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Each router creates Type 1 LSA to describe itself, its interfaces, and neighbors
LSA Type 7 is generated by ASBR inside an NSSA to describe routes redistributed into the NSSA
The main difference between LSA Type 7 and Type 5 is Type 7 is generated inside an NSSA while Type 5 is generated inside a standard area.
Below two cisco IOS commands will verify its OSPF neighbor relationship
-show ip ospf interface
-show ip ospf neighbor
The "show ip ospf" displays the number of times that the OSPF algorithm has been executed.
A stub area only allows LSA Type 1 , 2 and 3
Note: E2 is default external metric, but E1 is preferred over E2 if two equal-cost paths exist.
ABR will inject a default route to totally stubby and stub area
=Stub area replaces LSA Type 5 (External LSA - created by an ASBR to advertise network from the autonomous system) with a default route.
=totally stubby area replaces both LSA Type 5 and LSA type 3 (Summary LSA - created by an ABR to advertise network from other areas, but still within the AS, sometimes called inter-area routes) with a default route.
Three restrictions apply to OSPF stub areas.
-No virtual links are allowed.
-The area cannot be backbone area
-No Autonomous system border routers are allowed.
Two routers will become neighbors when they agree on the following :
Area-id, authentication, hello and dead intervals, stub area flag.
All OSPF routers in stubby areas can have type 3 LSAs in their database.
All OSPF routers in NSSA areas can have type 3 and type 7 LSAs in their database.
The "show ip ospf neighbor" command can be used to view the current state of the OSPF database loading process.
By default, FR is classified as a non-broadcast network, meaning it doesn't send any broadcasts/multicasts like RIP, OSPF or EIGRP updates across the network(hello packets of OSPF are multicast to 224.0.0.5). hence, in NBMA network, the neighbors are not discovered automatically, they must be configured manually. There are two ways to simulate a broadcast model on a NBMA network.
=Define a network type as broadcast with the "ip ospf network broadcast" interface sub-command.
=Configure the neighbor
Besides these two ways, another way of making OSPF work with FR is configuring the network as Point-to-point network (or
Point-to-multipoint, note that OSPF treats point-to-multipoint network as a collective of points to points links).
Note: point-to-multipoint networks do not maintain a DR/BDR relationship.
The 224.0.0.5 address is the multicast for OSPF routers.
The command "ip ospf network point-to-multipoint" configures this interface as point-to-multipoint for broadcast media. In broadcast networks, there is no need to specify neighbors. No DR or BDR is elected in this network type.
Note: IF we configure the interface as a point-to-point non-broadcast ( with the "ip ospf network point-to-multipoint non-broadcast" command) then we need to manually declare the OSPF neighbors.
commands that can be used to view neighbor adjacencies.
- show ip ospf neighbors
- show ip ospf interfaces
BGP
====
The command "show ip bgp summary" displays the IBGP and EBGP neighbors that are configured.
For EBGP, the next hop is always the IP address of the neighbor specified in the neighbor command. For IBGP, the protocol states that the next hop advertised by EBGP should be carried into IBGP.
The two reasons for the appearance of 0.0.0.0 as the next hop for a network in the show ip bgp command is
- The network was originated via redistribution of an interior gateway protocol into BGP.
- The network was originated via a network or aggregate command.
A rule of aggregation is "Aggregation applies only to routes that exist in the BGP routing table". An aggregated route is forwarded if at least one more specific route of the aggregation exists in the BGP routing table.
The BGP command "show ip bgp" provides the RID, local preference, next hop, and BGP path
Route decision process in BGP
=============================
-prefer highest weight (local to router)
-prefer highest local preference (global within AS)
-prefer route originated by the local router (next hop = 0.0.0.0)
-prefer shortest AS path
-prefer lowest origin code (IGP < EGP)
-prefer lowest MED (exchanged between autonomous system)
-prefer EBGP path over IBGP path
-prefer the path through the closet IGP neighbor (IGP cost)
-prefer oldest route for EBGP paths.
-prefer the path with the lowest neighbor BGP router-id
-prefer the path with the lowest neighbor IP address.
The eBGP multihop allows a neighbor connection between two external peers that do not have a direct connection. The multi-hop is only for eBGP and not for iBGP.
IBGP routers need not be directly connected.
The BGP split-horizon rule states that a route learned from one IBGP neighbor will not be advertised to another IBGP neighbor.
The "network" statement in other routing protocols (EIGRP, OSPF, RIP..) is used to enable routing protocol on the interfaces within that "network" statement. But in BGP, the function of a network statement is to tell the router to search the IP routing table for a particular network, and if that network is found, originate it into the BGP database.But notice that there must be an exact match in the routing table to appear the network in the BGP table.
Conditions that can cause BGP neighbor establishment to fail
=============================================================
-There is an access list blocking all TCP traffic between the two BGP neighbors.
-The BGP neighbor is referencing an incorrect autonomous system number in its neighbor statement.
BGP routers have enormous routing tables so it uses much memory to proceed these routes. When a BGP policy is changed, the BGP session needs to be reset for the policy to take effect. But the resetting results in route churn and route flapping. There are two ways to clear a BGP session without resetting the TCP session between them.
- Soft reset:
stores all received(inbound)routing policy updates without modification in a table so that when a new filter is applied, the router will use this table to calculate changes without resetting the TCP session between the two BGP peers. This is memory-intensive method and is not recommended.
-Route-refresh:
allows a BGP router to request a remote peer resend its BGP adj. This allows the BGP router to re-apply the inbound policy. The route-refresh capability requires no extra memory on the local router.
Note: If the routes were redistributed into BGP from an IGP , then we will see the next hop of 0.0.0.0
By default, the community attributes are not advertised to BGP neighbors.
The show ip bgp command is used to display entries in the BGP routing table.
MED(Multi-exit-discriminator) is used when we have multiple entry points (connections) to another AS. A lower MED value is preferred over a higher value. Notice that the comparison between the MED only occurs if the first AS is the same in two (or more) paths.
Note: An entry of 0.0.0.0 in the "Next hop" indicates that the router has some non-BGP routed to this network.
Being PBR is configured, the set command can specify
- all interfaces through which the packets can be routed.
- adjacent next hop router in the path toward the destination
- type of service and precedence in the IP packets.
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